This invention relates to a process for chemically reducing acrylonitrile (AN) monomer in nitrile polymers and more particularly to packaging materials shaped from the polymer wherein free AN monomer therein is minimal.
Thermoplastic nitrile polymer compositions containing polymerized AN can be shaped into a wide variety of useful articles by conventional techniques such as extrusion, milling, molding, drawing, blowing, etc. Applications for such shaped articles are widespread and include structural units where properties such as stiffness, impact resistance, dimensional stability, high gloss and toughness are required, for example pipe, bars, appliance housings, car parts, refrigerator liners and the like, replacements for glass for weight reduction as for automotive lenses, layer(s) in laminated windshields, security panels and/or other multi-layer structural laminates. When the polymerized AN level is at least about 50 weight percent, the compositions uniquely exhibit excellent solvent resistance and low permeability to liquids and gases which make them especially useful as a lightweight substitute for glass in packaging and particularly in the manufacture of bottles, film, sheet, tubs, cups, trays and other containers for liquids and solids.
In manufacturing nitrile polymer compositions, it is well known and has been tolerated that trace levels of free, unconverted AN monomer remain absorbed within the polymer particles when polymerization is not 100% complete and which is therefore present in products formed therefrom. Recently, however, certain government regulatory agencies are moving toward establishing maximum permissible levels of AN monomer in the environment on grounds that excess AN in the atmosphere generated during handling and working of these resins may constitute a health hazard in view of the toxicity of AN monomer. Regulations in particular have been applied to packaging materials intended for contact with environmentally sensitive products such as food, beverages, pharmaceuticals, cosmetics and the like for which application nitrile polymers are especially suited.
Efforts to reduce free, unreacted AN in nitrile polymers in response to such government pronouncements have involved steam stripping the polymer before melt processing, and when carried out for a sufficient time this can greatly reduce the residual monomer to miniscule levels. However, even with polymer initially having extremely low residual AN content, it was unexpectedly discovered that free AN monomer is thermally regenerated due to polymer unzipping up to levels on the order of 10 times the initial level during melting at standard melt processing times and temperatures. Such an increase caused an associated increase in extractable AN from shaped packaging materials in contact wih food simulating solvents. Accordingly, reduction of AN monomer in the raw polymer proved unsuccessful in minimizing AN in shaped products formed in an operation involving melt processing.